Sorry to necro, but I was looking at this and was hoping to get more clarification.

What's the rule backing up this claim that you only get one instance of fortification? I know there's the general rule that bonuses of the same type don't stack, but the description of fortification says nothing about a bonus at all, let alone a particular type. Is there some other rule saying that you can only have one instance of such an effect, even when they're coming from different sources?

Sorry to necro, but I was looking at this and was hoping to get more clarification.

What's the rule backing up this claim that you only get one instance of fortification? I know there's the general rule that bonuses of the same type don't stack, but the description of fortification says nothing about a bonus at all, let alone a particular type. Is there some other rule saying that you can only have one instance of such an effect, even when they're coming from different sources?

The game's general stacking rules cover this I beleive. When you have two instances of the same thing in varying degree's, the better value counts.

So if you have Fortification 25% and 50% the 50% counts.
If you have a ring of fire resistance 20 and 30, the 30 counts.
If your wearing bracers of Armor +8 and Studded Leather armor, the bracers count for your AC.

The game's general stacking rules cover this I beleive. When you have two instances of the same thing in varying degree's, the better value counts.

I was about to ask what you were basing this on, since all I could think of was the same-type bonuses issue (which fails to apply to fortification), but while trying to articulate myself I was reminded of this:

PRD wrote:

Same Effect More than Once in Different Strengths: In cases when two or more identical spells are operating in the same area or on the same target, but at different strengths, only the one with the highest strength applies.